1. Field of the Invention
The present invention generally relates to a diagnostic or an inspection apparatus utilizing an ultrasonic echo signal, and particularly relates to an input circuit system for receiving a signal from a piezoelectric transducer.
2. Description of the Prior Art
In such a diagnostic or an inspection apparatus utilizing an ultrasonic echo signal as typically represented by an ultrasonic diagnostic apparatus, it is required to provide various technical contrivances in a circuit for receiving a reception signal from a piezoelectric transducer. That is, for example, a parallel resonance circuit is arranged by connecting an inductor in parallel to a piezoelectric transducer in order to compensate the reduction in reception voltage at an input terminal of a reception amplifier due to bypassing of the reception signal through an inter-electrode capacity of the piezoelectric transducer and an equivalent capacity of a cable existing in parallel to the inter-electrode capacity. Such compensation or a matching circuit therefor, and an effect thereof are described, for example, in an article by R. N. Thruston; "Effect of Electrical and Mechanical Terminating Resistances on Loss and Bandwidth According to the Conventional Equivalent Circuit of a Piezoelectric Transducer" in IRE Transactions on Ultrasonics Engineering (1960): an article by J. W. Hunt et al.; "Ultrasound Transducers for Pulse - Echo Medical Imaging" in IEEE Transactions on Medical Engineering BME 30, No. 8 (1983), etc.
These conventional input circuits employ such a voltage detection system in which a load resistor is connected to an input terminal of a reception amplifier so that a voltage appearing across the load resistor is amplified by the reception amplifier. Therefore, in view of sensitivity of detection, it is desirable that the load resistance as well as the impedance provided in parallel to the former are made sufficiently high. However, if the Q of the resonance circuit is made larger than the mechanical Q of the piezoelectric transducer, the duration of pulses becomes long while the amplitude thereof is increased at the terminal of the load resistor, so that the resolution is deteriorated. Accordingly, the Q of the resonance circuit is limited within a predetermined range and therefore the load resistance can not be made sufficiently high. Consequently, the effect of the compensation circuit is limited correspondingly.
Further, in such a conventional input circuit of the voltage detection system, it is necessary to provide an inductor and therefore the conventional input circuit is not suitable to be formed into an integrated circuit.